Conventionally, in optical communication and optical measurement fields, there has been broadly used a waveguide modulator in which a light waveguide and modulation electrodes are formed on a substrate having electro-optic effect. In a light modulator using a substrate having electro-optic effect as described in Patent Document 1 or 2, there has been a method of suppressing occurrence of resonance phenomenon in which microwaves applied to the light modulator resonates in the substrate and occurrence of pyro-electric phenomenon in the substrate by forming a conductive film shown in FIG. 1 on the side surface and the rear surface of the substrate so as to be grounded. In addition, it is possible to improve electric characteristics (S11, S21) and suppress bias jump and the like. S11 is defined as reflection characteristics of electric signals that can be obtained by measuring a reflection amount at the time when an outer terminal gives the input signals to a signal electrode. S21 is defined as transmission characteristics of electric signals that can be obtained by measuring a transmission amount of output signals with respect to the input signals inputted from the outer terminal to the signal electrode. Generally, dip phenomenon can be checked in the transmission characteristics (S21). In addition, in case of X plate, the bias jump is phenomenon where electric charges are generated on the substrate by pyro-electric phenomenon, and the electric charges have an adverse effect on the electrodes formed on the substrate. In addition, FIG. 1 is a sectional view illustrating a light modulator. The reference numeral 1 represents the substrate, the reference numeral 2 represents the light waveguide, the reference numeral 3 represents a buffer layer, the reference numeral 4 represents a signal electrode, the reference numeral 5 represents an earth electrode, and the reference numeral 6 represents the conductive film. Hereinafter, the signal electrode 4 and the earth electrode 5 are referred to as modulation electrodes.
Patent Document 1 PCT Japanese Translation Patent Publication No. H5-509415
Patent Document 2 Japanese Examined Patent Application Publication No. 2919132
Meanwhile, to implement the light modulator having broadband, there has been a method that the substrate is formed with a thickness of several tens of μm thinner than the known thickness of 500 μm so as to satisfy velocity matching condition between microwaves and light waves and reduce a drive voltage. For example, in the following Patent Document 3 or 4, there is a method that a light waveguide and modulation electrodes are formed on a thin substrate having a thickness of 30 μm or less, the thin substrate is bonded to the other substrate having low permittivity, velocity matching between microwaves and light waves is performed by lowering effective refractive index thereof with respect to the microwaves, and mechanical strength thereof is maintained.
Patent Document 3: Japanese Unexamined Patent Application Publication No. S64-18121
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-215519
However, when the above-described conductive film is formed on the substrate that is formed so as to have a very small thickness (hereinafter, it is referred to as ‘thin plate’), an area of the side surface of the thin plate excessively decreases since the thickness of the substrate is thin. Thus, there has been a problem that adhesive strength between the substrate and the conductive film is weak and the conductive film is easily exfoliated from the substrate. In addition, when the substrate is formed thin, optical confinement in the light waveguide deteriorates, and thus a phenomenon where a light distribution pattern of the light waves is spread in a thickness direction of the thin plate or a transverse direction (which is a direction perpendicular to the thickness direction of the thin plate). When the conductive film is formed on the rear surface of the substrate, there has been a problem that the light waves are scattered or absorbed by the conductive film.